Because of the taste aldehyde imparts to liquids stored in polyethylene terephthalate containers, it has been attempted to remove the aldehyde from polyethylene terephthalate resin from which such containers are made. Heating under reduced pressure has been tried but it has not been possible to remove all of the aldehyde from the resin by this method. Hydroxyethyl groups in polyethylene terephthalate are known to break down when exposed to heat and form acetaldehyde. Then new aldehyde is formed and aldehyde that may be removed by heat and reduced pressure is replaced by aldehyde newly generated and the resin is not purified.